Distributor plates (or a plurality of adjacently disposed distributor plates) in a synthetic fiber spin pack in the form of thin sheets in which polymer distribution flow paths are etched (e.g., photochemically) to provide precisely formed and densely packed passage configurations are well known from U.S. Pat. Nos. 5,162,074 and 5,344,297 each issued in the name of William H. Hills (the entire content of each being expressly incorporated hereinto by reference, and hereinafter referred to as the "Hills patents"). The distribution flow paths may be shallow distribution channels arranged to conduct polymer flow along the distributor plate surface in a direction transverse to the net flow through the spin pack, and/or distribution apertures formed through the distributor plate.
In the photochemical etching process, one manner that could be envisioned in order to reduce internal spin pack pressures is to enlarge the depth of the polymer distribution channels which are photochemically etched into the plates. However, while increasing the depth of the polymer distribution channels is one possible solution to potentially excessive spin pack pressures, there is a practical limitation to the depth of such channels which is possible with current technology. In this regard, the photochemically etched holes not only penetrate into the depth of the plate, but also extend sideways (e.g., parallel to the plate surface). This leads to the through holes being larger in diameter when formed than is ideal and the resulting placement of the polymer flow to be less accurate. This is particularly critical in multicomponent spin packs where the photochemically etched plate may be used to precisely position the multiple polymer flows to create a pattern or shape to the fiber cross-section (e.g., the formation of a trilobal sheath-core bicomponent fiber where it is desirable to have a uniform thickness to the sheath polymer all around the trilobal cross-section).
Currently, therefore either high spin pack pressure drops are tolerated (with the potential for production difficulties, such as leakage) or more than one photochemically etched plate (using a thin plate for the precise holes and a relatively thicker plate (or plates) for the distribution channels is used. Use of multiple plates, however, increases the difficulties in assembling spin packs, increases inventory difficulties and/or may be more expensive. Moreover, multiple plates increase the opportunity for the plates to incorrectly align thereby leading to potentially severe processing problems.
Therefore, what has been needed in this art are improved thin photochemically etched plates usefully employed in spin packs, but which minimize (if not eliminate entirely) potentially excessive spin pack pressures. It is towards providing such improvements that the present invention is directed.
In a broad sense, the present invention is embodied in relatively thin (e.g., thickness of less than about 2.5 mm, and typically no greater than about 1.0 mm) photochemically etched plates for synthetic fiber-forming spin packs which include a metal layer exhibiting a relatively slow rate of photochemical etching properties (hereinafter "slow etch metal" or "SEM") and a layer of a metal layer exhibiting a relatively fast rate of photochemical etching properties (hereinafter "fast etch metal" or "FEM") which are adhered (laminated) to one another to form a composite substrate structure. The differential photochemical etch rates as between the SEM and FEM layers permit relatively dimensionally larger distribution channels and relatively dimensionally precise through holes to be formed in the composite substrate. In this regard, the FEM layer permits the formation via photochemical etching of dimensionally deeper and/or wider polymer distribution channels than is now possible with conventional photochemically etched spin pack plates. The SEM layer, on the other hand, allows for the formation of relatively dimensionally precise through holes via concurrent (simultaneous) photochemical etching with the FEM layer.